1. Field of the Invention
This invention relates to impact modification of thermoplastic polar polymers, such as poly(methacrylates), poly(vinyl chloride), polyamides, such as polycaprolactam, and the like, by a novel impact modifier, based on a multi-stage emulsion-prepared polymer wherein the second-stage polymer is found as domains within the rubbery crosslinked stage.
2. Description of the Prior Art
For more than 25 years, the plastics industry has sought to impact-modify thermoplastic resins. It has been known that impact modifiers based on acrylic rubbers, such as poly(butyl acrylate) offer reasonable toughness and heat stability. It has also been known that best results are observed when the acrylic rubber is crosslinked and also dispersed into relatively small particles within the thermoplastic matrix, and that this has been best accomplished by preparing a core/shell structure, wherein the core is the crosslinked acrylate rubber and the shell is a polymer, such as one based on methyl methacrylate, compatible with the matrix to be modified. The shell has further been modified to incorporate functional groups, such as acid, capable of reacting with certain functional groups in the matrix polymer, such as amine end groups in polyamides.
These efforts have been commercially successful, but the need still exists for improvements in the balance of impact strength, melt flow, and other properties whilst retaining the matrix properties of heat distortion temperature and hardness. The present invention relates to thermoplastic polar polymers modified with acrylic multi-stage impact modifiers for which the structure differs from the conventional core-shell structure which is commonly found in commercial toughened polar polymers.
Little is taught in the prior art relevant to such impact modifiers of high acrylic ester content. Falk et al., U.S. Pat. No.4,473,679, teach a multi-stage core/shell composition having a rubbery acrylic shell. Novak, U.S. Pat. No. 4,474,927, teaches a multi-stage polymer of an acrylate rubber core and a functionalized acrylate rubber shell, the shell containing acid groups, for modification of polyamides. Peascoe et al, European Patent Application 295,562, teach a crosslinked (meth)acrylate ester rubbery phase and an interpenetrating crosslinked styrenic resin phase useful in compatibilization of certain thermoplastics. Frankel et al., U.S. Pat. No. 4,814,373, herein incorporated by reference, teach elastomeric acrylic gumstocks which have similar structural features to the impact modifiers taught herein, but do not teach the utility of such structures as impact modifiers. Koller et al., U.S. Pat. No. 5,066,708, teach polymers similar to those of Frankel et al. useful as damping compositions in blends with thermosettable non-polar elastomers.
There exist a number of patents and papers which address the morphology which can be obtained in multi-stage emulsion polymerization. In general, these claim either a core/shell structure or an interpenetrating network of the two phases, at least one phase being cross-linked. There has been no showing that the "domain" structure taught herein produces useful impact modifiers. Rather, the core/shell polymers are described as being impact-imparting rubbers encased in a hard shell which enables ease of isolation by spray- drying or coagulation and also compatibility with the matrix to be impact-modified.
Owens, U.S. Pat. No. 3,808,180, herein incorporated by reference, gives an excellent description of emulsifiers, graft-linking monomers, cross-linking monomers, initiators, and the like, useful in making staged (meth)acrylic emulsion polymers, although he does not directly teach the impact modified blends of the present invention.